RENO, Nev. – Magma moving deep below Lake Tahoe apparently triggered an unprecedented swarm of 1,600 tiny earthquakes during a seven-month period, but they stopped in February and there's no cause for alarm, experts said yesterday.

The migration of the molten rock material 20 miles beneath the surface of the Sierra Nevada also probably caused the mountain beneath the Mount Rose Ski resort southwest of Reno to rise 8 millimeters, or about 3/8 of an inch, researchers said.

"We've been watching earthquakes for 30 years in the Tahoe area and have never witnessed an earthquake swarm anything like this," said Ken Smith, a research seismologist at the University of Nevada, Reno.

"The magma is the force breaking the rock and that's what we see the earthquakes associated with," he said yesterday during an interview.

"This is going to help us better understand how earthquakes develop in the first place," he said.

The deep earthquakes 25 miles southwest of Reno forced several miles of rock to spread apart by 1 meter but were of no greater magnitude than 2.2. They occurred from August 2003 to February 2004, then stopped.

Since then, no deep quakes have been recorded and there has been no notable movement of Slide Mountain at the Global Positioning System station atop the mountain, home of the Mt. Rose Ski Resort on the main highway from Reno to Lake Tahoe, Smith said.

Smith is the lead author of an article on the findings that will be published in the August edition of the journal Science. The article, "Evidence for Deep Magma Injection Beneath Lake Tahoe, Nevada-California," was posted on the journal's Science Express Web site yesterday.

Smith and other article co-authors emphasized the findings speak more to emerging technology that enables scientists to detect such minute changes in the earth than an increased threat of earthquakes along the Sierra's eastern front.

"We don't see any concern from this. It's so deep, about 20 miles beneath the surface," Smith said. That's twice as deep as the deepest earthquakes recorded previously in the area since measurements began 30 years ago, he said.

The study marks the first time that techniques used to measure seismology have been teamed up with satellite technology to pinpoint such activity in the Sierra.

Researchers said the findings should provide a boost to a new federal program aimed at installing hundreds of GPS stations and seismic recording stations nationwide.

The northern Sierra's volcanic history dates back 30 million years, but that period ended about 3 million years ago, Smith said. Since then, the uplifting of tectonic plates has dominated the region's geology, so much so the Sierra range is moving to the northwest at a rate of 12 to 14 millimeters a year, he said.

"The oldest volcanoes in this area are 1 to 2 million years old. Right now, the Sierras are essentially popping up like a cork," Smith said.

Michael Reichle, acting California state geologist, said state experts will be "keeping a close eye on future seismic activity in the Tahoe area."

The most recent instances of magma reaching the surface in the Lake Tahoe area occurred about 1 million years ago.